Silica glass and its manufacturing method

ABSTRACT

A method is provided for manufacturing a silica glass that is substantially free of chlorine. The method includes the step of separately expelling a silicon tetrafluoride gas, a combustion gas, and a combustible gas from a burner made of silica glass, the flow velocity of the silicon tetrafluoride gas being within the range of about 9 slm/cm 2  to about 20 slm/cm 2 . The method further includes the steps of producing minute silica glass particles by reacting the silicon tetrafluoride gas with water produced by a reaction of the combustion gas with the combustible gas, depositing the minute silica glass particles on a target, and producing the silica glass by fusing and vitrifying the minute silica glass particles deposited on the target.

This application is a continuation-in-part of U.S. application Ser. No.08/915,562, filed Aug. 21, 1997, now U.S. Pat. No. 5,958,809.

This application claims the benefit of Japanese Application No.09-246841, filed in Japan on Sep. 11, 1997, which is hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to silica glass that is substantially freeof chlorine (Cl) and has a superior durability against ultraviolet (UV)light, and its method of manufacture.

2. Discussion of the Related Art

Conventionally, reduction projection exposure apparatus called a stepperis often used for exposing and transcribing integrated circuit patternsonto a wafer, such as a silicon wafer, using photolithographictechnology.

The optical system of this stepper (reduction projection exposureapparatus) is mainly constructed of an illumination optical system and aprojection optical system. The illumination optical system is used, toradiate the light source beam uniformly onto a reticle where anintegrated circuit pattern is drawn. The projection optical system isused to reduce the image of the integrated circuit pattern, and projectand transfer the reduced image onto the wafer.

Due to recent trends towards greater integration of LSI, it has becomenecessary for the pattern transcribed on the wafer to have a higherresolution. Therefore, as the light source of the stepper, shorterwavelengths, from the g-line (436 nm) and the i-line (365 nm) to excimerlasers, such as KrF (248 nm) and ArF (193 nm), are increasingly beingused.

Conventional optical glass used as the lens for these illuminationoptical systems and projection optical systems has a problem of its lowlight transmittance in the short wavelength range below the i-line.Currently, synthesized silica glass is used as the lens material insteadof conventional optical glass.

This silica glass can be manufactured (synthesized) for example, byvapor-phase synthesis called the “direct method”. This direct methodinclude the following steps, for example.

(1) The step for expelling a gaseous silicon compound as a material, anoxygen gas, and a hydrogen gas from a burner made of silica glass. Ingeneral, the gaseous silicon compound is diluted with a carrier gas (anoxygen gas, for example) upon emission.

(2) The step for generating minute silica glass particles (soot) byreacting the gaseous silicon compound and water which is a reactionproduct of the oxygen and hydrogen gases.

(3) The step for depositing the minute silica glass particles on atarget.

(4) The step for creating silica glass (lump) by fusing and vitrifyingthe minute silica glass particles deposited on the target.

It has been discovered that the silica glass manufactured with thismanufacturing method using silicon tetrafluoride gas as a gaseoussilicon compound is substantially free of chlorine in the silica glassand exhibits a high durability against ultraviolet light, as comparedwith silica glass manufactured using silicon tetrachloride as thematerial.

However, when silica glass is synthesized using the silicontetrafluoride (SiF₄) gas as the material, a new problem emerges as atrade-off for the high durability to ultraviolet light. The problem isthat the uniformity in refractive index tends to deteriorate in theresultant silica glass.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a silica glass and itsmanufacturing method that substantially obviate the problems due tolimitations and disadvantages of the related art.

An object of the present invention is to provide a silica glass having asuperior ultraviolet light durability and a uniform refractive indexprofile.

Another object of the present invention is to provide a method formanufacturing a silica glass having a superior ultraviolet lightdurability and a uniform refractive index profile.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, the presentinvention provides a silica glass that is substantially free of chlorine(Cl), including a fluorine (F) concentration of about 100 ppm to about450 ppm, a difference between the maximum refractive index and theminimum refractive index of the silica glass (Δn) being within the rangeof about 1.0×10⁻⁷ to about 1.0×10⁻⁵.

In another aspect, the present invention provides a silica glass havinga chlorine concentration of about 0.1 ppm or less and a fluorineconcentration of about 100 ppm to about 450 ppm, the silica glass havinga substantially uniform refractive index distribution with respect

to ultraviolet light such that a difference between the maximumrefractive index and the minimum refractive index is within the range ofabout 1.0×10⁻⁷ to about 1.0×10⁻⁵.

In another aspect, the present invention provides a method formanufacturing a silica glass that is substantially free of chlorine, themethod including the steps of separately expelling a silicontetrafluoride gas, a combustion gas, and a combustible gas from a burnermade of silica glass, the flow velocity of the silicon tetrafluoride gasbeing within the range of about 9 slm/cm² to about 20 slm/cm²; producingminute silica glass particles by reacting the silicon tetrafluoride gaswith water produced by a reaction of the combustion gas with thecombustible gas; depositing the minute silica glass particles on atarget; and producing the silica glass by fusing and vitrifying theminute silica glass particles deposited on the target.

In a further aspect, the present invention provides a method formanufacturing a silica glass, the method including the steps ofexpelling a combustion gas, a combustible gas, and a material gasincluding a silicon tetrafluoride gas separately from a burner, the flowvelocity of the silicon tetrafluoride gas included in the material gasbeing within the range of about 9 slm/cm² to about 20 slm/cm²; reactingthe combustible gas with the combustion gas to produce water; reactingthe silicon tetrafluoride gas with the water to produce and depositminute silica particles on a target; and fusing and vitrifying theminute silica particles deposited on the target to produce the silicaglass.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constituteapart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 shows a cross-section of the nozzles of a gas burner made ofsilica glass according to the present invention; and

FIG. 2 is a graph showing the correlation between the flow velocity atwhich a silicon tetrafluoride (SiF₄) gas is expelled and the refractiveindex difference (Δn) of the resultant silica glasses.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some of the causes of the above-mentioned problems are inappropriatemanufacturing conditions during the synthesis of the silica glass usingthe direct method. Examples of such include an inconsistent temperaturedistribution on the synthesis surface (target surface) formed by flame,fluctuations in flame hydrolysis reaction or thermolysis/thermaloxidation reaction, and nonuniform diffusion of impurities in the silicaglass (such as the OH group or chlorine). These variations(fluctuations) in the manufacturing conditions have significantinfluences on striations that may be grown in the silica glass (called“striae”) and the refractive index profile in the radial direction inthe silica glass. These fluctuations cause non-uniformity in therefractive index of the silica glass. In addition, it has been foundthat the poor uniformity in the refractive index of the silica glassmanifests itself especially when the silicon tetrafluoride gas is usedas the material for the silica glass synthesis.

As a result of diligent studies, the inventors of the present inventionhave identified the cause of lowering the uniformity in the refractiveindex of the synthesized silica glass in the case of using the silicontetrafluoride gas. The cause is that these manufacturing conditionsfluctuate during the silica glass synthesis, and accordingly, thenon-reacted silicon tetrafluoride that could not be hydrolyzed isincluded in the silica glass. Therefore, the density (distribution) offluorine in the silica glass may excessively increase and/or anon-uniform fluorine density distribution may be created.

Thus, the present inventors have discovered that in order to obtainuniformity in the refractive index of the silica glass when silica glassis synthesized using silicon tetrafluoride, it is important to achieve auniform fluorine density distribution by controlling the fluorinedensity within a certain range,

Accordingly, the present invention provides a silica glass that issubstantially free of chlorine (Cl) in which the density of fluorine (F)is controlled within the range of about 100 ppm to about 450 ppm, and inwhich the difference between the maximum refractive index and theminimum refractive index (Δn) is controlled within the range of about1.0×10⁻⁷ to about 1.0×10⁻⁵.

Furthermore, in order to have a better uniformity in the refractiveindex of the silica glass, it is preferable to set the value of thefluorine density in the silica glass to be within the range of about 120ppm to about 300 ppm, and more preferably, the value should be withinthe range of about 140 ppm to about 200 ppm.

In addition, in the present invention, it is preferable to set the valueof the hydroxy group (OH group) density in the silica glass to be withinthe range of about 600 ppm to about 1300 ppm.

When the density of the hydroxy group in the silica glass is controlledwithin this range, it is possible to obtain a silica glass with asuperior ultraviolet light durability. This is because structuraldefects such as oxygen deficiency is compensated by the hydroxy group(OH group) and thus a stable structure is created.

For even better ultraviolet light durability for the silica glass, it ispreferable to have the hydroxy group density in the silica glass withinthe range from about 900 ppm to about 1200 ppm.

In the present invention, if the silica glass includes any one of metalimpurities, such as Mg (magnesium), Ca (calcium), Sc (scandium), Ti(titanium), V (vanadium), Cr (chromium), Mn (manganese), Fe (iron), Co(cobalt), Ni, (nickel), Cu (copper), Zn (zinc) and Al (aluminum), it ispreferable to set the density of such a metal element in the silicaglass to be about 20 ppb or less.

It has been found that each of these metal elements reduces thedurability against excimer laser irradiation of the silica glass.Therefore, by setting the density of each metal element to be about 20ppb or less, it is possible to obtain a silica glass with a superiordurability against excimer laser irradiation.

In another aspect, the present invention provides a method formanufacturing a silica glass that is substantially free of chlorine(Cl), including the steps of expelling a silicon tetrafluoride (SiF₄)gas, a combustion gas such as oxygen gas (O₂) and a combustible gas suchas hydrogen gas (H₂) separately from a burner made of silica glass;providing minute silica glass particles by reacting the silicontetrafluoride gas and water which is a reaction product of thecombustion gas (oxygen gas) and the combustible gas (hydrogen gas);depositing the minute silica glass particles on a target; and producingthe silica glass by fusing and vitrifying the minute silica glassparticles deposited on the target, wherein the flow velocity of saidsilicon tetrafluoride gas is within the range of about 9 slm/cm² toabout 20 slm/cm².

It was found that the flow velocity of the silicon tetrafluoride gas hasan influence on, for example, the density of the fluorine included inthe silica glass. Accordingly, by manufacturing the silica glass usingthe method described above, it is possible to control the fluorinedensity in the silica glass within the range of about 100 ppm to about450 ppm and to control the difference (Δn) between the maximumrefractive index and the minimum refractive index of the silica glasswithin the range of about 1.0×10⁻⁷ to about 1.0×10⁻⁵ with the silicaglass being essentially chlorine free.

Furthermore, from the point of view of easily controlling the fluorinedensity, the chlorine density, and the difference (Δn) between themaximum refractive index and the minimum refractive index of the silicaglass, it is more preferable to set the flow velocity of the silicontetrafluoride gas to be within the range of about 9.2 to about 19.0slm/cm².

In addition, for the operation of the manufacturing method of the silicaglass of the present invention, it is preferable to control the flowvelocity of the silicon tetrafluoride gas (SiF₄) using a mass flowcontroller. This is because by using a mass flow controller, it ispossible to control the flow velocity of the silicon tetrafluoride gasmore accurately, and accordingly, it is easy to control the fluorinedensity, the chlorine density, and the difference (Δn) between themaximum refractive index and the minimum refractive index of the silicaglass.

A mass flow controller is a type of mass flow meter and is a flow meterthat is capable of controlling the flow of gas material by monitoringthe mass of the fluid that passes through the mass flow controller whichin this case is the gaseous state of a gas material such as silicontetrafluoride gas, or liquid state material obtained by lowering thetemperature of the gas material.

For the operation of the manufacturing method of the silica glass of thepresent invention, it is preferable that minute silica glass particles(soot) are deposited on the target while the target is being lowered ata rate (lowering rate) within the range of about 0.5 mm/hr to about 2.35mm/hr so that a predetermined distance is maintained between the uppersurface of the ingot and the burner. This is preferable because bycontrolling the lowering speed of the target within the above range, itis possible to easily control the fluorine density, the chlorinedensity, and the difference (Δn) between the maximum refractive indexand the minimum refractive index of the silica glass.

In the manufacturing method of the silica glass of the presentinvention, it is preferable that the fluorine (F) density in the silicaglass be within the range of about 100 ppm to about 450 ppm and that thedifference (Δn) between the maximum refractive index and the minimumrefractive index of the silica glass be within the range of about1.0×10⁻⁷ to about 1.0×10⁻⁵. This is because by controlling the fluorinedensity in the silica glass and the difference (Δn) between the maximumrefractive index and the minimum refractive index of the silica glass insuch a manner, it is possible to obtain a silica glass with a superiorultraviolet light durability and a uniform refractive index profile.

Furthermore, to obtain a silica glass with an even better ultravioletlight durability and uniform refractive index and also to furthersimplify the control of the manufacturing process and quality control ofthe material, it is more preferable that the difference (Δn) between themaximum refractive index and the minimum refractive index of the silicaglass be within the range of about 5.0×10⁻⁷ to about 6.0×10⁻⁶.

In addition, for the operation of the manufacturing method of the silicaglass of the present invention it is preferable that the ratio of theflow of the combustion gas, such as oxygen gas (O₂), to the flow of thecombustible gas, such as hydrogen gas (H₂), (oxygen gas flow/hydrogengas flow) be within the range of about 0.2 to about 0.5. This is becauseby controlling the ratio of the flow of the combustion gas to the flowof the combustible gas to be within this range, it is possible to easilycontrol the density of the hydroxy group (OH group) in the silica gaswithin the range of about 600 ppm to about 1300 ppm.

Here, the flow of the combustion gas is the total flow of combustion gas(such as oxygen gas) and in the case that the gas is expelled fromseparate expelling tubes, it corresponds to the sum of the flows of allthe combustion gases emitted from expelling tubes. Also, when the samecombustion gas is used as a carrier gas for the silicon tetrafluoridegas, the flow of that combustion gas is included in the total flow ofthe combustion gas defined here.

Similarly, the flow of the combustible gas (such as hydrogen gas) is thetotal flow of combustible gases, and when the gases are expelled fromseparate expelling tubes, the “flow of the combustible gas” correspondsto the sum of the flows of all the combustible gases emitted from theexpelling tubes.

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

First Preferred Embodiment (Working Example)

Various silica glasses were manufactured (synthesized) using themanufacturing method of the present invention.

(1) Using a burner made of silica glass, a silicon tetrafluoride (SiF₄)gas, an oxygen (O₂) gas, and a hydrogen (H₂) gas were supplied byexpelling each of the gases at respective predetermined flow rates.

The silicon tetrafluoride gas was diluted with a carrier gas (oxygengas: flow rate 1.8 slm), and was expelled from the center tube of aburner made of silica glass that is controlled by a mass flow controllerwith a flow rate set at 1.50 slm (See Table 1). The silicontetrafluoride gas used had a purity of 99.99% or more. Contamination ofmetal impurities was 10 ppb or less for Fe and 2 ppb or less for each ofNi and Cr.

The burner that was used is described with reference to FIG. 1. FIG. 1shows a cross-section of the nozzle of the burner made of silica glass.A material tube 12 with an inner diameter of 4.5 mm (also referred to asa center tube or the first tube) is provided at the center of nozzle 10of the silica glass burner for expelling material. In FIG. 1, the innerdiameter of the material tube 12 is indicated by reference character t0.

In addition, at the periphery of the material tube 12, the second tube14 is arranged in a concentric configuration. Oxygen gas was expelled ata rate of 22 slm from a gap 24 (1.0 mm) between the material tube 12 andthe second tube 14. The gap between the material tube 12 and the secondtube 14 is indicated by reference character t1 in FIG. 1.

At the periphery of the second tube 14, a third tube 16 is arranged in aconcentric configuration. The gap 26 between the second tube 14 and thethird tube 16 is 1.0 mm. In FIG. 1, the gap 26 between the second tube14 and the third tube 16 is indicated by reference character t2, and t2and t1 have the same values. From the gap 26 between the second tube 14and the third tube 16, a hydrogen gas was expelled at a rate of 75 slm.

On the outside of the third tube 16, a fourth tube 18 is arranged at a45 mm distance in a concentric configuration with respect to tubes 12 to16. In the gap 28 (45 mm) between the third tube 16 and the fourth tube18, twenty two (22) fifth tubes 20 each having a diameter of 6.0 mm arearranged at appropriate intervals. A hydrogen gas was expelled from gap28 between third tube 16 and fourth tube 18 (the gap 28 is indicatedwith a reference character t3), and an oxygen gas was expelled from thefifth tubes 20.

As described above, the reason why the oxygen and hydrogen gases areexpelled from separate nozzles (for example, for oxygen gas, from gap 24between second tube 12 and third tube 14, and from the fifth tubes 20)is that the oxygen gas and hydrogen gas will then react with each othermore uniformly.

The ratio of the oxygen gas flow to the hydrogen gas flow affects thedensity of fluorine (F) and the density of hydroxy groups (OH groups) inthe resulted silica glass, and accordingly, in this first embodiment,the ratio of the oxygen gas flow to hydrogen gas flow (oxygen gasflow/hydrogen gas flow) was set to 0.4.

The speed at which the silicon tetrafluoride (SiF₄) gas was expelledfrom the center tube of the silica glass burner (also referred to as theflow velocity or material flow speed) has a dominant effect on thefluorine (F) density in the silica glass. The speed at which the silicontetrafluoride (SiF₄) gas is expelled can be obtained by dividing theflow rate of the silicon tetrafluoride gas by the area of the tip of thematerial tube of the silica glass burner. Therefore, when the innerdiameter of the material tube is constant, the speed at which thematerial is expelled from the tip of the burner is equal to the materialflow speed of the silicon tetrafluoride in the tube. In the firstembodiment, as shown in Table 1, the speed at which the silicontetrafluoride (SiF₄) gas is expelled was accurately controlled using amass flow controller to provide a constant flow velocity of 9.4 slm/cm².

(2) Next, minute silica glass particles (soot) were produced by reactingthe silicon tetrafluoride (SiF₄) gas with water which was a reactionproduct of the oxygen gas and the hydrogen gas. That is, using a silicaglass burner, the oxygen gas and the hydrogen gas were mixed and burned,and in the resultant combustion flame, minute silica glass particlesmade of SiO₂ and hydrogen fluoride (HP) were produced by the hydrolysisreaction indicated in formula (1) below.

SiF₄+2H₂O→SiO₂+4HF  (1)

This hydrolysis reaction occurs until the minute silica glass particlesreach the deposition surface (stacking surface) on the target. Most ofthe silicon tetrafluoride (SiF₄) gas is hydrolyzed and becomes thematerial for producing the minute silica glass particles. However, aportion of the silicon tetrafluoride gas that is not hydrolyzed isincluded in the minute silica glass particles. Therefore, the silicontetrafluoride gas included in the minute silica glass particles withoutbeing hydrolyzed may increase the fluorine density and become one of thecauses for the poor uniformity in the refractive index of the resultantsilica glass.

(3) Next, minute silica glass particles (soot) were deposited on thetarget. While the minute silica glass particles (soot) were deposited onthe target, the target was lowered at a constant speed so that thedistance between the deposition surface (stacking surface) of the minutesilica glass particles and the silica glass burner was kept to be nearlyconsistent. In this first embodiment, as shown in Table 1, the speed atwhich the target was lowered was set to 1.00 mm/hr. The distance betweenthe deposition surface (stacking surface) and the burner made of silicaglass is set to be approximately 300 mm.

In the first embodiment, in order to uniformly deposit minute silicaglass particles on the target, the target was rotated at a rate of sevenrotations per minute and was rocked at 90 second interval within a 80 mmmovement range, for example.

(4) Finally, silica glass was created by heating, fusing, and vitrifyingthe minute silica glass particles (soot) deposited on the target. Theheat used for fusing and vitrifying the minute silica glass particlecomes from the combustion of the oxygen and hydrogen gases expelled fromthe silica glass burner.

Surrounding the target, fire-resistant materials made of alumina with apurity of 99% were arranged with an inner shape having the followingdimensions: length 600 mm×width 800 mm×height 800 mm. In this and thefollowing examples, silica glass was deposited on the target while thetarget was rotated, and the produced silica glasses each took the formof an ingot with a diameter between 180 mm to 240 mm.

As described above, during the manufacture of synthetic silica glassusing the direct method, minute silica glass particles are produced by aflame hydrolysis reaction, reach the target, and are fused and vitrifiedto form a silica glass ingot.

The “silica glass” in the present invention includes ingots, silicaglass material (partially finished product) cut out from the ingot, andsilica glass parts (such as a lens) obtained by processing the silicaglass material.

Evaluating Properties of the Manufactured Silica Glass

(1) Measurement of the Refractive Index

First, seven test pieces (having a diameter of 150 mm and a thickness of50 mm; referred to as the test pieces for the refractive indexmeasurement) were cut out from the center portion of the manufacturedsilica glass ingot in order to measure the difference (Δn) between themaximum refractive index and the minimum refractive index in the silicaglass.

Next, for the purpose of removing the residual distortion(birefringence), the cut out test pieces for the refractive indexmeasurement were annealed in a furnace that was maintained atatmospheric conditions at a temperature of 1000° C. for 10 hours.Subsequently, the temperature of each test piece for the refractiveindex measurement was lowered to 500° C. with a lowering speed of 10°C./hour and then was returned to room temperature by naturalair-cooling.

The refractive indices of the test pieces for the refractive indexmeasurement, which went through the thermal process above, were measuredby a Fizeau interferometer using a He—Ne laser as a light source. Thesame measurements were conducted on all the seven test pieces and thedifference in the refractive index (Δn) was calculated using the maximumvalue and the minimum value of the measured refractive indices. Theresults are shown in Table 1.

As understood from Table 1, the value of Δn, which indicates theuniformity in the refractive index of the silica glass ingot of thefirst embodiment is 4.3×10⁻⁶. This value is within the desirable rangeof An (about 5.0×10⁻⁷ to about 6.0×10⁻⁶) and therefore, the refractiveindex difference (Δn) that is acceptable for the optical parts of theoptical system of excimer laser lithography apparatus (about 1.0×10−7 toabout 1.0×10⁻⁵) was achieved.

FIG. 2 shows data for the refractive index differences (Δn) measured forsecond and third embodiments and second to fourth comparative examples,which will be described below. As shown in FIG. 2, there is acorrelation between the speed at which silicon tetrafluoride (SiF₄) gasis expelled and the refractive index difference (Δn) in the resultantsilica glasses. The X axis represents the material flow speed (flowvelocity) (slm/cm²) of silicon tetrafluoride gas, and the Y axisrepresents the refractive index difference (Δn).

As shown in FIG. 2, the greater flow velocity of silicon tetrafluoridegas is, the greater refractive index difference (Δn) becomes. When theflow velocity is within the range of about 9 to about 22 slm/cm², thedifference in the refractive index (Δn) between the maximum and theminimum in the silica glass becomes about 1.0×10⁻⁵ or less. As describedabove, when the refractive index difference (Δn) has a value of 1.0×10⁻⁵or less, it is possible to achieve a uniform refractive index profilethat is acceptable for use in optical parts of the optical system forexcimer laser lithography apparatus.

(2) Measurement of Fluorine (F) Density

A test piece for the fluorine (F) density measurement was cut out in arectangular block shape (length 20 mm×width 20 mm×thickness 10 mm) fromthe center portion of the silica glass ingot. Then, after the test piecefor the fluorine (F) density measurement was fused with sodium carbonateand made into a specific amount, a quantitative analysis of the fluorinedensity in the silica glass was conducted using ion-chromatographyanalysis.

The results also are shown in Table 1. As shown in Table 1, the fluorinedensity in the silica glass ingot of the first embodiment is less than200 ppm. Thus, the silica glass has a lower fluorine density value thanthose in the comparative examples and the density is within thepreferable range for the fluorine density (about 100 to about 450 ppm).

Accordingly, the manufacturing method of the first embodiment provides aconsistent fluorine density while maintaining the uniformity in therefractive index of the silica glass ingot, thereby improving thedurability against ultraviolet light of the silica glass.

(3) Measurement of the Hydroxy Group (OH) Density

A test piece for the hydroxy group (OH) density measurement was cut outin the shape of a rectangular block (length 20 mm×width 20 mm×thickness10 mm) from a center portion of the silica glass ingot. Then, afteroptical polishing was applied to both sides of the test piece thedensity of the hydroxy groups was measured using an infrared absorptionspectroscopy by measuring the amount of infrared light absorbed by theOH groups at a wavelength of 1.38 μm.

The results are shown in Table 1. As shown in Table 1, the density ofthe hydroxy group in the silica glass ingot of the first embodiment was980 ppm. This value is within the preferable range of the hydroxy groupdensity (about 600 to about 1300 ppm). Accordingly, the manufacturingmethod of the first embodiment provides a desirable density of thehydroxy groups, which improves the durability against ultraviolet lightof the silica glass.

(4) Measurement of the Density of Metal Elements

A test piece for the metal element measurement was cut out in arectangular block shape (length 20 mm×width 20 mm×thickness 10 mm) froma center portion of the silica glass. Then the density of each of themetal elements (Mg, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Al)was measured using an inductive coupling plasma.

The results are shown in Table 2. As shown in Table 2, the density ofeach metal element in the silica glass ingot of the first embodiment hadan extremely low value of 20 ppb or less. Therefore, it was confirmedthat with the manufacturing method of the first embodiment, it ispossible to greatly reduce the density of each metal element (Mg, Ca,Se, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Al) in the silica glass thatis considered to have adverse effects on the durability against laserbeam irradiation. Thus, with the manufacturing method of the firstembodiment, the density of each metal element is reduced, and thedurability against excimer laser beams is improved.

(5) Measurements of Chlorine, Na and K Densities

A test piece for the chlorine, Na and K density measurements was cut outin a rectangular block shape (length 20 mm×width 20 mm×thickness 10 mm)from a center portion of the silica glass input. Then, the chlorine, Na,and K densities of the test piece were measured using radioactivationanalysis by thermal neutron irradiation.

The results are shown in Table 2. As shown in Table 2, the chlorine (Cl)density of the silica glass ingot was below the detection limit (≦0.1ppm), the Na density was below the detection limit (≦1 ppb), and the Kdensity also was below the detection limit (≦50 ppb). Therefore, it wasconfirmed that with the manufacturing method of the first embodiment, itis possible to greatly reduce the chlorine, Na and K densities in thesilica glass, which are considered to have adverse effects on therefractive index of the silica glass. Therefore, by using

the manufacturing method of the first embodiment, the durability againstexcimer laser irradiation can be improved as a result of reducing thechlorine, Na and K densities.

(6) Measurement of the Durability Against Ultraviolet Light

A test piece for the ultraviolet light resistance (ArF excimer laser)property measurement was cut out in a circular column shape (diameter 60mm, thickness 10 mm) from the center portion of the silica glass ingot.Then, precision polishing was applied to the test piece using anabrasive on the two opposiing surfaces (circlular shape) of the testpiece so that the test piece had a parallelness of 10 seconds or less, aflatness of 3 or less newton rings, and a surface roughness of rms=10 Å.The final thickness of the test piece was 10±0.1 mm. In order to removeany residual abrasive, a finishing process was applied to the test piecewith high purity SiO₂ powders.

First, the bulk transmittance of the resulting test piece for theultraviolet light durability measurement was measured using aspectrophotometer before irradiation with an ArF excimer laser beam. Theresults are shown in Table 1. As shown in Table 1, the bulk absorptioncoefficient at a wavelength of 193 nm is 0.001 cm⁻¹ or less. This valuecorresponds to a bulk transmittance of 99.9% per unit centimeter, whichis an excellent value. The bulk absorption coefficient was calculatedbased on the following equation (2). $\begin{matrix}{{{Bulk}\quad {absorption}\quad {coefficient}} = {- \frac{\ln \quad \left( {{{Transmittance}/{Theoretical}}\quad {transmittance}} \right)}{{Thickness}\quad {of}\quad {the}\quad {test}\quad {piece}}}} & (2)\end{matrix}$

Here, the theoretical transmittance is a transmittance determined by thereflection loss at the surface of the test piece and the internalscattering loss, assuming that the internal absorption loss of thetransmitting beam equals to 0.

Next, in order to precisely measure the durability against ultravioletlight of the silica glass, a dehydrogenation process was applied to eachof the seven test pieces. Each test piece was placed in a heat treatingfurnace made of an anhydrous (free of OH group) silica glass tube havingan inner diameter of 110 mm and a length of 1000 mm. The pressure in theheat treating furnace was reduced to 10⁻⁵ torr using a diffusion pumpand then dissolved hydrogen was removed by maintaining each test piecein the heat treating furnace at a temperature of 700° C. for 60 hours(vacuum anneal). Then, the beat treating furnace and each of the testpieces (seven pieces) were cooled to room temperature to complete thedehydrogenation processing of the test pieces (seven pieces).

The measurement of the dissolved hydrogen density was conducted using alaser Raman spectrophotometer. For all of the test pieces (sevenpieces), the density of the dissolved hydrogen was below the detectionlimit (1×10¹⁶ molecules/cm³). Thus, it was confirmed that all of theseven test pieces were fully dehydrogenized.

For all test pieces, the Raman beam intensity at a wavelength of 606cm⁻¹ was not changed by the dehydrogenation process. Therefore, it isconsidered that only the dissolved hydrogen was removed from the silicaglass by the dehydrogenation process, and that the structure of thesilica glass itself did not change.

The saturated transmittance and bulk absorption coefficient (thetransmittance and the bulk absorption coefficient after changes in thesequantities due to irradiation of the silica glass with many ArF excimerlaser pulses are saturated) were measured for each of the test piecesthat went through the dehydrogenation process. An ArF excimer laser beamused had unit pulse energy density of 200 mJ/cm²/pulse, a repetitivepulse frequency of 100 Hz, and the number of pulses of about 3×10⁵ toabout 5×10⁶.

The result are shown in Table 1, the transmittance at the saturationpoint with a wavelength of 193 nm was 81.0%/cm, and the bulk absorptioncoefficient was 0.115 cm⁻¹. From this result, it was confirmed that thesilica glass of the first preferred embodiment has a superiortransmittance for ultraviolet light even after subjected to severeconditions (the above dehydrogenation process.)

When the ArF excimer laser beam irradiated the silica glass that did notgo through the above dehydrogenation process (the silica glass containssome dissolved hydrogen) for unit pulse energy density of 200mJ/cm²/pulse, a repetitive pulse frequency of 100 Hz and the number ofpulses of about 1×10⁶, the bulk transmittance at a wavelength of 193 nmwas 99.0%/cm and the bulk absorption coefficient was 0.01 cm⁻¹.

Therefore, it was confirmed that the silica glass of the presentembodiment has a superior durability aginst ultraviolet irradiation.

TABLE 1 First Second Third Compara- Compara- Compara- Compara- Embodi-Embodi- Embodi- tive tive tive tive ment ment ment Example 1 Example 2Example 3 Example 4 Flow (slm) 1.50 2.64 3.00 1.32 3.95 4.50 5.27 Flowvelocity 9.4 16.6 18.9 8.3 24.8 28.3 33.1 (slm/cm²) Lowering speed 1.001.97 2.20 — 2.40 3.00 3.93 (mm/hr) Ingot diameter 180 180 200 — 240 220210 Difference in 4.3 × 10⁻⁶ 5.3 × 10⁻⁶ 5.8 × 10⁻⁶ — 1.5 × 10⁻⁵ 3.0 ×10⁻⁵ 5.0 × 10⁻⁵ refractive index Δn F density (ppm) 150 160 190 — 500750 1150 OH density (ppm) 980 1180 1020 — 900 1130 1000 Initial bulk0.001 or 0.001 or 0.001 or — 0.001 or 0.001 or 0.001 or absorption lessless less less less less coefficient (cm⁻¹) Initial bulk 99.9 or 99.9 or99.9 or — 99.9 or 99.9 or 99.9 or transmittance greater greater greatergreater greater greater (%/cm) Saturated bulk 0.115 0.121 0.117 — 0.1130.116 0.111 absorption coefficient (cm⁻¹) saturated 81.0 80.6 80.8 —81.2 80.9 81.3 transmittance (%/cm) —: not measurable

TABLE 2 First Second Third Embodiment Embodiment Embodiment Cl density(ppm) 0.1 or less  0.1 or less  0.1 or less  Na density (ppb) 1.0 orless  1.0 or less  1.0 or less  K density (ppb) 50 or less 50 or less 50or less Mg density (ppb) 20 or less 20 or less 20 or less Ca density(ppb) 20 or less 20 or less 20 or less Sc density (ppb) 20 or less 20 orless 20 or less Ti density (ppb) 20 or less 20 or less 20 or less Vdensity (ppb) 20 or less 20 or less 20 or less Cr density (ppb) 20 orless 20 or less 20 or less Mn density (ppb) 20 or less 20 or less 20 orless Fe density (ppb) 20 or less 20 or less 20 or less Co density (ppb)20 or less 20 or less 20 or less Ni density (ppb) 20 or less 20 or less20 or less Cu density (ppb) 20 or less 20 or less 20 or less Zn density(ppb) 20 or less 20 or less 20 or less Al density (ppb) 20 or less 20 orless 20 or less

Second Preferred Embodiment Manufacture of Silica Glass

In the embodiment 2, the flow amount of silicon tetrafluoride (SF₄) gaswas increased from 1.50 slm of the first embodiment to 2.64 slm. Alongwith this, the speed at which silicon A tetrafluoride gas was expelledwas increased from 9.4 slm/cm² to 16.6 slm/cm². These were the onlychanges in the conditions under which the silica glass was manufactured(synthesized).

Property Evaluation of the Silica Glass

In the second preferred embodiment, under the same conditions as thefirst preferred embodiment, (1) measurement of the refractive index, (2)measurement of the fluorine (F) density, (3) measurement of the hydroxygroup (OH) density, (4) measurement of the density of the metalelements, (5) measurement of the density of chlorine, Na and K, and (6)measurement of the durablity against ultraviolet light were conducted.

The results are shown in Tables 1 and 2. As shown in Table 1, the valueof Δn, which indicates the uniformity of the refractive index of thesilica glass ingot of the second preferred embodiment was 5.3×10⁻⁶.Thus, it was possible to achieve the characteristics (1.0×10⁻⁵) for therefractive index difference at a quality level that is acceptable forthe optical parts of an optical system for excimer laser lithographyapparatus. In other words, it was confirmed that in accordance with themanufacturing method of the second preferred embodiment, it is possibleto manufacture a silica glass with a superior uniformity in therefractive index.

Also, as shown in Table 1, the fluorine density of the silica glass ofthe second preferred embodiment was 160 ppm. It was confirmed that thefluorine density was slightly greater than that of the silica glass ofthe first preferred embodiment. Therefore, it is considered that withthe manufacturing method of the second preferred embodiment, thefluorine (F) density in the desirable range can be easily obtained, andthe durability against ultraviolet light of the silica glass is improvedwhile maintaining the uniformity in the refractive index of the silicaglass.

In addition, as shown in Table 1, the density of the hydroxy groups inthe silica glass ingot of the second preferred embodiment was 1180 ppm.

Moreover, the densities of the metal elements (Mg, Ca, Sc, Ti, V, Cr,MA, Fe, Co, Ni, Cu, Zn, Al) each were an extremely low value of below 20ppb, as shown in Table 2.

Therefore, with the manufacturing method of the second preferredembodiment, it is considered that the superior uniformity in therefractive index of the silica glass was further achieved by loweringthe density of each metal element.

As shown in Table 2, the chlorine (Cl) density in the silica glass ofthe second preferred embodiment was below the detection limit (0.1 ppm).The Na density was below the detection limit (1 ppb) and the K densitywas below the detection limit (50 ppb) as well.

As shown in Table 1, an excellent value of 0.001 cm⁻¹ or less wasobtained for the bulk absorption coefficient before irradiation by anArF excimer laser beam (initial bulk absorption coefficient) and thebulk transmittance (initial bulk transmittance) was 99.9%/cm or greaterfor the silica glass ingot of the second preferred embodiment.

The saturated bulk absorption coefficient after irradiation by an ArFexcimer laser beam (bulk absorption coefficient at the saturation point)of the silica glass of the second preferred embodiment, to which thedehydrogenation process was applied as in the first preferredembodiment, was 0.121 cm⁻¹ and the saturated bulk transmittance(transmittance at saturation point) was 80.6%/cm. Therefore, it wasconfirmed that even after subjected to severe conditions (theabove-mentioned dehydrogenation process), the silica glass of the secondpreferred embodiment has a superior transmittance for ultraviolet light,which indicates a superior durability against ultraviolet light.

Third Preferred Embodiment Manufacture of Silica Glass

In the third preferred embodiment, the silicon tetrafluoride (SiF₄) gasflow amount was increased from 1.50 slm for the first preferredembodiment to 3.00 slm. Along with this, the speed at which silicontetrafluoride gas was expelled was increased from 9.4 slm/cm² to 18.9slm/cm². With the exception of the ingot diameter of the silica glassbeing changed from 180 mm of the first preferred embodiment to 200 mm,this was the only change in the conditions under which the silica glasswas manufactured (synthesized).

Property Evaluation of the Silica Glass

For the third preferred embodiment, under the same conditions as thefirst preferred embodiment, (1) measurement of the refractive index, (2)measurement of the fluorine (F) density, (3) measurement of the hydroxygroup (OH) density, (4) measurement of the density of the metalelements, (5) measurement of the density of chlorine, Na, and K, and (6)measurement of the durability against ultraviolet light were alsoconducted.

The results are shown in Tables 1 and 2. As shown in Table 1, the valueof Δn, which indicates the uniformity in the refractive index of thesilica glass ingot of the third preferred embodiment was 5.8×10⁻⁶. Thus,it was possible to achieve a quality level for the refractive indexdifference that is acceptable for the optical parts of an optical systemfor excimer laser lithography apparatus. Therefore, it was confirmedthat in accordance with the manufacturing method of the third preferredembodiment, it is possible to manufacture a silica glass with a superioruniformity in the refractive index.

As shown in Table 1, it was confirmed that the fluorine density of thesilica glass of the third preferred embodiment was 190 ppm. Therefore,in accordance with the manufacturing method of the third preferredembodiment, a fluorine density within a specific range can easily beobtained. With the results of the refractive index, it becomes possibleto improve the durability against ultraviolet light while maintainingthe uniformity in the refractive index of the silica glass.

As shown in Table 1, the hydroxy group density of the silica glass ofthe third preferred embodiment was 1020 ppm. Therefore, in accordancewith the manufacturing method of the third preferred embodiment, it ispossible to obtaine the hydroxy group density within the desirablerange.

Furthermore, as shown in Table 2, the density of each of the metalelements (Mg, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Al) in thesilica glass of the third preferred embodiment was the extremely lowvalue of 20 ppb or less.

Moreover, as shown in Table 2, the chlorine (Cl) density in the silicaglass of the third preferred embodiment was below the detection limit(0.1 ppm). The Na density was below the detection limit (1 ppb) and theK density also was below the detection limit (50 ppb).

As shown in Table 1, the excellent value of 0.001 cm⁻¹ or less wasobtained for the bulk absorption coefficient (initial bulk absorptioncoefficient) and the bulk transmittance (initial bulk transmittance) was99.90%/cm or greater before irradiation of the silica glass ingot of thethird preferred embodiment by an ArF excimer laser beam.

The saturated bulk absorption coefficient after irradiation by an ArFexcimer laser beam (bulk absorption coefficient at saturation point) ofthe silica glass of the third preferred embodiment, to which thedehydrogenation process was applied as in the the first preferredembodiment, was 0.117 cm⁻¹, and the bulk transmittance was 80.8%/cm.Therefore, it was confirmed that even after being subjected to severeconditions (the above-mentioned dehydrogenation process), the silicaglass of the third preferred embodiment had a superior durabilityagainst ultraviolet light irradiation.

COMPARATIVE EXAMPLE 1

In the comparative example 1, the flow amount for silicon tetrafluoride(SiF₄) gas was decreased from 1.50 slm for the first preferredembodiment to 1.32 slm. Along with this, the speed at which silicontetrafluoride gas was expelled was decreased from 9.4 slm/cm² to 8.3slm/cm². This was the only change under which the silica glass wasmanufactured (synthesized).

However, under the manufacturing conditions of the comparative example1, it was impossible to grow silica glass on the target and to take itout as an ingot. Therefore, eavluation of properties of the silicaglass, such as measurement of the refractive index, could not beconducted.

COMPARATIVE EXAMPLE 2 Manufacture of Silica Glass

In the comparative example 2, the flow amount of silicon tetrafluoride(SiF₄) gas was increased from 1.50 slm for the first preferredembodiment to 3.95 slm. Along with this, the speed at which silicontetrafluoride gas was expelled was increased from 9.4 slm/cm² to 24.8slm/cm². With the exception of the ingot diameter of the silica glassbeing changed from 180 mm of the first preferred embodiment to 240 mm,this was the only change in the conditions under which the silica glasswas manufactured (synthesized).

Property Evaluation of the Silica Glass

For the comparative example 2, under the same conditions as the firstpreferred embodiment, (1) measurement of the refractive index, (2)measurement of the fluorine (F) density, (3) measurement of the hydroxygroup (OH) density, and (6) measurement of the durability againstultraviolet light were also conducted.

The results are shown in Table 1. As shown in Table 1, the value Δn,which indicates the uniformity of the refractive index of the silicaglass ingot of the comparative example 2 was 1.5×10⁻⁵. This was aninsufficient quality level for the characteristics of the refractiveindex difference to be acceptable for the optical parts of the opticalsystem for excimer laser lithography apparatus.

In addition, as shown in Table 1, it was found that the fluorine densityof the silica glass of the comparative example 2 was 500 ppm. Therefore,with the manufacturing method of the comparative example 2, the fluorinedensity exceeds the preferable range for the fluorine (F) density (about100 to about 450 ppm). It is considered that the refractive index of thesilica glass becomes non-uniform due to this undesirable flurinedensity.

Similarly, as shown in Table 1, the hydroxy group (OH) density in thesilica glass of the comparative example 2 was 900 ppm. As a density forthe hydroxy group this value is slightly low. However, it was found thateven with the manufacturing method of the comparative example 2, it ispossible to obtain a silica glass with a hydroxy group density withinthe designated range (about 600 to about 1300 ppm).

The excellent value of 0.001 cm⁻¹ or less was obtained for the bulkabsorption coefficient (initial bulk absorption coefficient) of thesilica glass ingot of the comparative example 2 and the bulktransmittance (initial bulk transmittance) was 99.9%/cm or greaterbefore irradiation by an ArF excimer laser beam.

The saturated bulk absorption coefficient after irradiation by an ArFexcimer laser beam (bulk absorption coefficient at a saturation point)of the silica glass of the comparative example 2, to which thedehydrogenation process was applied as in the first preferredembodiment, was 0.113 cm⁻¹ and the saturated bulk transmittance was81.2%/cm. Thus, it was confirmed that the durability against ultravioletlight of the silica glass of the comparative example 2 was comparable tothat for the silica glass ingots of the first to third embodiments.

COMPARATIVE EXAMPLE 3 Manufacture of Silica Glass

In the comparative example 3, the flow amount of silicon tetrafluoride(SiF₄) gas was increased from 1.50 slm for the first preferredembodiment to 4.50 slm. Along with this, the speed at which silicontetrafluoride gas was expelled was increased from 9.4 slm/cm² to 28.3slm/cm². With the exception of the ingot diameter of the silica glassbeing changed from 180 mm of the first preferred embodiment to 220 mm,this was the only change in the conditions under which the the silicaglass was manufactured (synthesized).

Property Evaluation of the Silica Glass

For the comparative example 3, under the same conditions as the firstpreferred embodiment, (1) measurement of the refractive index, (2)measurement of the fluorine (F) density, (3) measurement of the hydroxygroup (OH) density, and (6) measurement of the durability againstultraviolet light were also conducted.

The results are shown in Table 1. As shown in Table 1, the value Δn,which indicates the uniformity of the refractive index in the silicaglass ingot of the comparative example 3 was 3.0×10⁻⁵. Thus, the silicaglass of the comparative example 3 has an insufficient quality level forthe characteristics of the refractive index difference for it to beacceptable for the optical system (optical parts) for excimer laserlithography apparatus.

As shown in Table 1, it was found that the fluorine density in thesilica glass of the comparative example 3 was 750 ppm. Therefore, in themanufacturing method of the comparative example 3, the fluorine densityexceeds the desirable range (about 100 to about 450 ppm), and it isconsidered that the refractive index of the silica glass becomesnon-uniform due to this undesirable flurine density.

As shown in Table 1, the density of the hydroxy group in the silicaglass of the comparative example 3 was 1130 ppm. Therefore, it was foundthat even with the manufacturing method of the comparative example 3, itis possible to obtain a silica glass with a hydroxy group density withinthe designated range (about 600 to about 1300 ppm.)

The excellent value of 0.001 cm⁻¹ or less was obtained for the bulkabsorption coefficient (initial bulk absorption coefficient) of thesilica glass ingot of the comparative example 3 and the bulktransmittance (initial bulk transmittance) was 99.9%/cm or greaterbefore irradiation by an ArF excimer laser beam. There was nosignificant difference observed from these values of the first to thirdpreferred embodiments.

The saturated bulk absorption coefficient after irradiation by an ArFexcimer laser beam (bulk absorption coefficient at saturation point) ofthe silica glass of the comparative example 3, to which thedehydrogenation process was applied as in the first preferredembodiment, was 0.116 cm⁻¹ and the saturated bulk transmittance(transmittance at the saturation point) was 80.9%/cm. Therefore, it wasconfirmed that for the durability against ultraviolet light, the silicaglass of the comparative example 3 has an comparable value to the silicaglass ingot of the first to third embodiments.

COMPARATIVE EXAMPLE 4 Manufacture of Silica Glass

In the comparative example 4, the flow amount of silicon tetrafluoride(SiF₄) gas was increased from 1.50 slm for the first preferredembodiment to 5.27 slm. Along with this, the speed at which silicontetrafluoride gas was expelled was increased from 9.4 slm/cm² to 33.1slm/cm². With the exception of the ingot diameter of the silica glassbeing changed from 180 mm of the first preferred embodiment to 210 mm,this was the only change in the conditions under which the silica glasswas manufactured (synthesized).

Property Evaluation of the Silica Glass

For the comparative example 4, under the same conditions as in the firstpreferred embodiment, (1) measurement of the refractive index, (2)measurement of the fluorine (F) density, (3) measurement of the hydroxygroup (OH) density, and (6) measurement of the durability againstultraviolet light were also conducted.

The results are shown in Table 1. As shown in Table 1, the value A,which indicates the uniformity in the refractive index of the silicaglass ingot of the comparative example 4 was 5.0×10⁻⁵. Thus, the silicaglass of the comparative example 4 had insufficient quality for thecharacteristics (uniformity) of the refractive index difference to beacceptable for the optical system (optical parts) of an excimer laserlithography apparatus.

In addition, as shown in Table 1, it was confirmed that the fluorinedensity of the silica glass of the comparative example 4 was 1150 ppm.Therefore, in the manufacturing method of the comparative example 4, thefluorine density exceeds the preferable range (about 100 to about 450ppm) and it is considered that the refractive index of the silica glassingot becomes non-uniform due to this undesirable fluorine density.

As shown in Table 1, the density of the hydroxy group in the silicaglass of the comparative example 4 was 1000 ppm. Therefore, it was foundthat even with the manufacturing method of the comparative example 4, itis possible to obtain a silica glass with a hydroxy group density withinthe designated range (about 600 to about 1300 ppm.)

The excellent value of 0.001 cm⁻¹ or less was obtained for the bulkabsorption coefficient (initial bulk absorption coefficient) of thesilica glass ingot of the comparative example 4, and the bulktransmittance (initial bulk transmittance) was 99.9%/cm or greaterbefore irradiation by an ArF excimer laser beam. There was nosignificant difference observed from these values for the first to thirdpreferred embodiments.

The saturated bulk absorption coefficient (bulk absorption coefficientat the saturation point) after irradiation by an ArF excimer laser beamof the silica glass of the comparative example 4, to which thedehydrogenation process was applied as in the first preferredembodiment, was 0.111 cm⁻¹ and the saturated bulk transmittance was81.3%/cm. Therefore, it was confirmed that for the durability againstultraviolet light, the silica glass of the comparative example 4 showsan comparable value to the silica glass ingot of the first to thirdprefered embodiments.

As described above, according to the present invention, by setting thefluorine density in a substantially chlorine (Cl) free silica glass towithin a range of about 100 ppm to about 450 ppm and by limitting thedifference between the maximum refractive index and the minimumrefractive index of the silica glass to within the range of about1.0×10⁻⁷ to about 1.0×10⁻⁵, the problem of non-uniform refractive indexprofile when the silica glass is manufactured using silicontetrafluoride gas as a material is solved. Thus, it is possible toobtain silica glass appropriate for optical parts that requires a highdurability against ultraviolet light, such as optical system (opticalparts) for photolithography apparatus using an ultraviolet laser havinga wavelengh of 300 nm or less (ArF excimer laser, for example).

A manufacturing method of silica glass that is substantially free ofchlorine (Cl) according to the present invention includes the steps ofexpelling each of silicon tetrafluoride (SiF₄), a combustion gas, and acombustible gas separately from a burner made of silica glass, andproducing minute silica glass particles by reacting the silicontetrafluoride gas and water which is a reaction product of the oxygengas and the hydrogen gas. The manufacturing method further includes thesteps of depositing the minute silica glass particles on a target, andproducing the silica glass by fusing and vitrificating the minute silicaglass particles deposited on the target. Here, the flow velocity of thesilicon tetrafluoride gas is preferably within the range of about 9 toabout 20 slm/cm² to easily control the fluorine density of the silicaglass to be within the range of about 100 to about 450 ppm and to easilycontrol the difference between the maximum refractive index and theminimum refractive index of the silica glass (Δn) to be within the rangeof about 1.0×10⁻⁷ to about 1.0×10⁻⁵. Accordingly, the present inventionsolves the problem that when the silicon tetrafluoride gas is used as amaterial for the synthesis of silica gas, the refractive index tends tobe non-uniform. Thus, according to the present invention, it becomespossible to easily and consistently obtain a silica glass that isappropriate for optical parts that require a high durability againstultraviolet light, such as the optical parts of an optical imagingsystem for photolithography apparatus which uses an ultraviolet laser of300 nm or less (ArF excimer laser, for example) as a light source.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the silica glass and itsmanufacturing method of the present invention without departing from thespirit or scope of the invention. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A silica glass member that is substantially freeof chlorine (Cl), and includes a fluorine (F) concentration of about 100ppm to about 450 ppm, wherein the silica glass member has a regionhaving dimensions of about 150 mm in diameter and about 50 mm inthickness, and a difference between the maximum refractive index and theminimum refractive index in the region (Δn) is within the range of about1.0×10⁻⁷ to about 1.0×10⁻⁵.
 2. The silica glass member according toclaim 1, wherein the concentration of each of metal elements, Mg, Ca,Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Al, in the silica glassmember is about 20 ppb or less.
 3. The silica glass member according toclaim 1, wherein the concentration of hydroxy (OH) groups in the silicaglass member is within the range of about 600 ppm to about 1300 ppm. 4.The silica glass member according to claim 3, wherein the concentrationof each of metal elements, Mg, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,Zn, and Al, in the silica glass member is about 20 ppb or less.
 5. Asilica glass member having a chlorine concentration of about 0.1 ppm orless and a fluorine concentration of about 100 ppm to about 450 ppm,wherein the silica glass member has a region having dimensions of about150 mm in diameter and about 50 mm in thickness, and has a substantiallyuniform refractive index distribution with respect to ultraviolet lightsuch that a difference between the maximum refractive index and theminimum refractive index in the region is within the range of about1.0×10⁻⁷ to about 1.0×10⁻⁵.
 6. The silica glass member according toclaim 5, wherein the concentration of each of metal elements, Mg, Ca,Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Al, in the silica glassmember is about 20 ppb or less.
 7. The silica glass member according toclaim 5, wherein the concentration of hydroxy groups in the silica glassmember is within the range of about 600 ppm to about 1300 ppm.
 8. Thesilica glass member according to claim 7, wherein the concentration ofhydroxy groups in the silica glass member is within the range of about900 ppm to about 1200 ppm.
 9. The silica glass member according to claim7, wherein the concentration of each of metal elements, Mg, Ca, Sc, Ti,V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Al, in the silica glass member isabout 20 ppb or less.
 10. The silica glass member according to claim 5,wherein the fluorine concentration is within the range of about 120 ppmto about 300 ppm.
 11. The silica glass member according to claim 10,wherein the fluorine concentration is within the range of about 140 ppmto about 200 ppm.
 12. The silica glass member according to claim 5,wherein the difference between the maximum refractive index and theminimum refractive index in the region is within the range of about5.0×10⁻⁷ to about 6.0×10⁻⁶.
 13. A silica glass ingot to be cut out toproduce a silica glass member of processing ultraviolet light, whereinthe silica glass ingot has a region having dimensions of about 150 mm indiameter and has a chlorine concentration of about 0.1 ppm or less and afluorine concentration of about 100 ppm to about 450 ppm, the silicaglass ingot having a substantially uniform refractive index distributionwith respect to ultraviolet light such that a difference between themaximum refractive index and the minimum refractive index in the regionis within the range of about 1.0×10⁻⁷ to about 1.0×10⁻⁵.
 14. A silicaglass member having a chlorine concentration of about 1.0 ppm or lessand a fluorine concentration of about 100 ppm to about 450 ppm, thesilica glass member having a substantially uniform refractive indexdistribution with respect to ultraviolet light such that a differencebetween the maximum refractive index and the minimum refractive index iswithin the range of about 1.0×10⁻⁷ to about 1.0×10⁻⁵ over dimensions ofabout 150 mm in diameter and about 50 mm in thickness.
 15. A silicaglass member that is substantially free of chlorine (Cl), and includes afluorine (F) concentration of about 100 ppm to about 450 ppm, whereinthe silica glass member has a region having dimensions of about 150 mmin diameter, and a difference between the maximum refractive index andthe minimum refractive index in the region (Δn) is within the range ofabout 1.0×10⁻⁷ to about 1.0×10⁻⁵.
 16. A silica glass lens that issubstantially free of chlorine (Cl), and includes a fluorine (F)concentration of about 100 ppm to about 450 ppm, wherein a differencebetween the maximum refractive index and the minimum refractive index inthe silica glass lens (Δn) over dimensions of about 150 mm in diameterand about 50 mm in thickness is within the range of about 1.0×10⁻⁷ toabout 1.0×10⁻⁵.
 17. A silica glass lens having a chlorine concentrationof about 0.1 ppm or less and a fluorine concentration of about 100 ppmto about 450 ppm, wherein the silica glass lens has a substantiallyuniform refractive index distribution with respect to ultraviolet lightsuch that a difference between the maximum refractive index and theminimum refractive index in the silica glass lens over dimensions ofabout 150 mm in diameter and about 50 mm in thickness is within therange of about 1.0×10⁻⁷ to about 1.0×10⁻⁵.
 18. A silica glass lens thatis substantially free of chlorine (Cl), and includes a fluorine (F)concentration of about 100 ppm to about 450 ppm, wherein a differencebetween the maximum refractive index and the minimum refractive index inthe silica glass lens (ΔN) is within the range of 1.0×10⁻⁷ to about1.0×10⁻⁵ over the dimension of about 150 mm in diameter.